Clean up the managed tunnel in v12 #26193

Follow-ups from the runtime tunnel stack (#25997, #25998, #25999, #26000, #26001) to clean up in v12.

• Drop the build-time tunnelRoute option in TanStack Start and Next.js, so the tunnel is only set up with serveTunnelIntegration. On Lambda, TanStack Start redirects every handler today because tunnelRoute turns the tunnel on after the runtime read _HANDLER.
• Decide what to do with the runtimes that have no runtime hook: Vercel Edge, Netlify Functions and Edge, Nitro's aws-lambda and netlify presets, and possibly Vercel Node. Either drop them, or find a way to support them. SvelteKit, Astro and Remix v3 fall back to their own request hook for now, while Nitro-based apps (Nuxt, SolidStart, TanStack Start) and React Router need handleTunnelRequestIfMatched there. See ref: Remove the framework tunnel hooks #26001 (comment).